1) Field of the Invention
The present invention relates to a multi-stage amplifier, and more particularly to a high-gain amplifier having a frequency band of more than 40 gigahertz.
2) Description of the Related Art
The rapid spread of the Internet has led to needs for communications systems with high transmission rate (e.g. 40 Gb/s) and high data-capacity. The communications systems require a high-gain amplifier having a frequency band of more than 40 gigahertz in a driver circuit for driving a modulator at a transmitter front end as well as in a preamplifier or an equalizer amplifier at a receiver front end.
Distributed amplifiers have a frequency band determinable from a ladder filter defined by the input capacitance of a transistor and the inductance (L) on a wire. Accordingly, they have been employed in the art as circuitry suitable for broadband applications. Among those, a cascode distributed amplifier includes a circuit which is connected to a common gate (or common base) transistor and exhibits a negative resistance in a high frequency band, and thus can amplify broadband signals. As a result, such a circuit currently becomes the mainstream gradually.
FIG. 1 is a circuit diagram illustrating the configuration of a conventional cascode distributed amplifier. The distributed amplifier includes a plurality of cascode amplifiers, each consisting of a transistor Q1 whose source is grounded and a transistor Q2 whose gate is grounded via a resistor 13 and an input capacitor 14, connected in parallel between an input line 11 and an output line 12. The gate of the transistor Q1 is connected to the input line 11 and the drain to the source of the transistor Q2. The drain of the transistor Q2 is connected to the output line 12.
The input line 11 has one end connected to an input terminal 15 and the other end grounded via a terminal resistor 16. The output line 12 has one end connected to an output terminal 17 and the other end grounded via a terminal resistor 18. In FIG. 1, the rectangular blocks denoted with the reference numerals 19 represent inductance components over the transmission lines, and the arrows (∇) indicate ground (earth) (they are similarly employed in other figures).
The transistor Q1 amplifies a signal input to the input terminal 15. The amplification band depends on the characteristics of a filter defined by the capacitance of the input capacitor 14 and a line inductance component (not shown) on the transistors Q1 and Q2 which are connected in serial. The amplification gain depends on the gain of the transistor Q1. Such amplification allows the transistor Q2 to exhibit a negative resistance in a high frequency band. The negative resistance increases the gain (so-called the gain jump) and accordingly prevents the gain from decreasing in a high frequency band.
The inventor has reported a cascode distributed amplifier with a frequency band of more than 40 gigahertz in “45-GHz distributed amplifier with a linear 6-Vp-p output for a 40 Gb/s LiNb 03 modulator driver circuit”, 2001 IEEE GaAs Digest, pp. 137, 2001 (hereinafter, “inventor's report”).
In general, however, the cascode distributed amplifier has a low gain. One approach to increase the gain is to connect plural amplifier stages, each including the cascode distributed amplifiers, in cascade. In this amplifier stages, if a pre-amplifier stage is direct current (hereinafter, “DC”) coupled to a post-amplifier stage, since each stage has a different bias level from another, it is difficult to determine the gain definitely. However, if the pre-amplifier stage is coupled to the post-amplifier stage via a DC cut capacitor, the gain can be determined definitely.
The capacitive coupling between stages requires a very large capacitor. This capacitor has an inductance component, which influences on the gain to be decreased in a high frequency range disadvantageously. In addition, both the distributed amplifier and the capacitor cannot be integrated on a single semiconductor substrate. Accordingly, it is necessary to connect the distributed amplifier integrated on a semiconductor substrate, to a capacitor externally located, by wiring. This arrangement causes the gain to be decreased in a high frequency range disadvantageously.